Present code division multiple access (CDMA) systems are characterized by simultaneous transmission of different data signals over a common channel by assigning each signal a unique code. This unique code is matched with a code of a selected receiver to determine the proper recipient of a data signal. These different data signals arrive at the receiver via multiple paths due to ground clutter and unpredictable signal reflection. Additive effects of these multiple data signals at the receiver may result in significant fading or variation in received signal strength. In general, this fading due to multiple data paths may be diminished by spreading the transmitted energy over a wide bandwidth. This wide bandwidth results in greatly reduced fading compared to narrow band transmission modes such as frequency division multiple access (FDMA) or time division multiple access (TDMA).
New standards are continually emerging for next generation wideband code division multiple access (WCDMA) communication systems as described in Provisional U.S. Patent Application No. 60/082,671, filed Apr. 22, 1998, and incorporated herein by reference. These WCDMA systems are' coherent communications systems with pilot symbol assisted channel estimation schemes. These pilot symbols are transmitted as quadrature phase shift keyed (QPSK) known data in predetermined time frames to any receivers within range. The frames may propagate in a discontinuous transmission (DTX) mode. For voice traffic, transmission of user data occurs when the user speaks, but no data symbol transmission occurs when the user is silent. Similarly for packet data, the user data may be transmitted only when packets are ready to be sent. The pilot symbols are equally spaced in time among sixteen equal time slots within the frame. The received pilot symbols may then be compared to the known pilot symbols to estimate and correct the magnitude and phase distortion of the received data signal.
A problem arises, however, when no data signals are included with the pilot symbols in a frame. Since there is nothing in the frame to indicate a presence or absence of data signals, each time slot of the frame must be treated as if data signals are present. Thus, a channel estimate must be completed for the frame, and the contents of each time slot must be decoded. A cyclic redundancy check (CRC) is used to determine if data is received correctly. If the CRC falls, the ratio of the decoded data power to the pilot power is then calculated to determine if data was transmitted (for the purpose of calculating the frame error rate). This unnecessary decoding, CRC computation, and power ratio estimation when no data is present, however, requires extra computation power. Furthermore, even algorithms which use both data and pilot symbols for channel estimation will not be reliable if it is not known if data is present.